A reflector used in combination with an interferometer and a laser light source provides a convenient means for determining a position reference for a body in space. However, if the body is capable of movement relative to the light source, the reflector must be designed to accommodate this. In particular, if the light source is fixed and the orientation of the body changes with respect to the light source, as with a rotary actuator arm in a data recording disk file, as the position of the body changes the incident light will strike different portions of the reflector, and the reflected light will have a reflected path that changes angle with respect to the incident light. As a result, detection of the reflected light requires additional optics or the ability to precisely move the interferometer.
In the prior art, this problem has been dealt with by using a corner cube type retro-reflector, which serves to reflect an incident light beam back to its source without regard to the angle of incidence. However, in cases where mass, size, and cost are important, the corner cube design is too bulky, large, and expensive to present a solution that is both practical and cost effective. Thus, there has existed an unmet need for an inexpensive and lightweight retro-reflector for attachment to a rotating body, capable of retro-reflecting an incident light beam without regard to the angle between the incident light beam and the rotating body.
An important application in which this need arises is servo writing for rotary actuator disk files. In this environment an external actuator position reference is required for writing precise servo patterns. According to the prior art, this reference is provided by a mechanical coupling to the rotary actuator or by placing a corner cube type retro-reflector on the rotary actuator. In either case, the drive may not be sealed during this operation due to the intrusion of the mechanical coupling or the presence of the corner cube, or alternatively, must be sealed using a special double sealed bearing in order to provide external access to the rotating mechanism. If the drive is not sealed, the servo write process must be conducted in a clean room environment, further adding to the expense of the drive.
Apart from servo writing, it would be advantageous in a rotary actuator disk file to provide head positioning information during operation by means of a fixed source light beam, an interferometer, and a retro-reflector attached to the rotary actuator arm. Previously, this has not been possible, due to the excessive size and weight of the corner cube type retro-reflector and a lack of an alternative means for retro-reflecting a fixed source light beam incident on an actuator arm without regard to the angle between the incident light beam and the actuator arm.